This invention relates to monitoring mechanisms, and in particular it concerns monitoring mechanisms for use in solvent recovery plants.
Many industrial processes use solvents, and often these solvents are discharged from the operating stages of the process in the form of an extremely diluted solvent-vapour-laden gas (the gas usually being air). For reasons of economy, or to avoid pollution, it is desirable to recover the solvents from the gas, and this can often be achieved by vapour-phase recovery using a material (for example, activated carbon) which will "strip" the solvent from the gas. The solvent is then recovered from the "stripping" material (in which it is present at concentrations considerably higher than it was in the discharged gas). In some instances a solvent-laden liquid may be discharged from an industrial process, and recovery of the solvent from the liquid may be effected by methods similar to those used in recovery from the gaseous phase. The recovery systems described hereinafter may be used to recover solvents from gases or liquids, but for convenience they refer only to recovery from solvent-laden gases.
One form of solvent recovery plant uses a two-stage recovery system. In the first stage, the solvent-laden gas is filtered, cooled or heated as necessary, and then passed through a bed of activated carbon, which "strips" the solvent from the gas; thus, the activated carbon gradually becomes loaded with solvent and will eventually extract no more solvent from the gas. The passage of the solvent-laden gas is then stopped, and, in the second stage of the recovery, steam is passed through the carbon beds to purge the solvent from the beds. The mixture of steam and solvent vapour so obtained is condensed, whereupon the liquid solvent and water so obtained is in most cases very easily separated.
In order for the solvent recovery to be a continuous process, a "two adsorber" recovery plant is used. While one adsorber is being charged with solvent, the other is being purged by steam, and when the bed of the first adsorber is loaded with solvent the solvent-laden gas supply is transferred to the second adsorber (the bed of which is now free of solvent), and the bed of the first adsorber purged with steam. This changeover procedure allows the solvent to be continuously extracted.
Control of the changeover of the functions of the adsorbers in a "two-adsorber" plant is important for efficient and economical running of the plant, and this control has previously been done in a number of ways. If the flowrate of solvent-laden gas into the system is steady, and the solvent-to-gas ratio is constant, the time for the bed to become loaded with solvent, which will be a constant, can be calculated, and the changeover of the adsorbers can be performed automatically or manually at fixed time intervals.
However, in practice the flowrate of gas into the recovery system may vary between wide limits, particularly if the system is taking, as its input, the outputs of solvent-laden gas from a number of sources. Nevertheless, if the solvent-to-gas ratio in the solvent-laden gas were constant then control could be effected by monitoring the flowrate of the incoming solvent-laden gas, and performing the changeover when a calculated amount of the solvent-laden gas had entered the recovery system. Unfortunately, however, in practice the solvent-to-gas ratio in the solvent-laden gas is very rarely constant and so this method too is of only very limited use.